Each year 12.7 million people worldwide are diagnosed with cancer and there are 7.6 million deaths from the disease.1 Radio-Therapy (RT) plays a pivotal role in the management of various types of cancer, since approximately 50% of all cancer cases being treated with this type of therapy. RT consists of the planning and accurate delivery of lethal doses of X-ray radiation to tumor tissues (i.e. within 1-2 mm) while sparing the surrounding healthy tissues. One of the key components in achieving these objectives is high quality imaging, as it is vital to visualize and localize the anatomy embedding the disease site. Without pinpointing the extent of cancerous tissues, the chances of hitting and killing the treatment targets are limited, leading to poor patient outcomes.
In recent years, advanced Image Guided Radio-Therapy (IGRT) techniques, aimed to guide the daily patient's treatment setup, have been proposed for clinical use. These IGRT techniques comprise: a) Cone-Beam Computed Tomography (CBCT) combined with a linear accelerator (linac), b) ultrasound imaging, c) tomotherapy, d) portal image verification relying on transmission images of bony anatomy or implanted fiducial markers, generated by the linac's Mega-Voltage (MV) beam, and e) Kilo-Voltage (kV) images produced by an X-ray tube mounted on the linac gantry. All of these imaging modalities are used separately except for kV and MV imaging which may be used together. The main benefits of using such imaging modalities include the reduction of safety margins leading to higher local tumor control and reduced normal tissue toxicity via safe dose escalation.
RT relies heavily on X-ray-based imaging for treatment guidance. X-ray transmission-based images (kV or MV) show great bony detail but are inherently limited in resolving soft-tissue organ structures. The ideal imaging technique for RT would feature the ability to: a) clearly distinguish soft-tissue structures (i.e. tumor target and organs) from the surrounding anatomical background, and b) capture and dynamically track tumor target and internal organ motion manifested as the change in shape and location of the organ volume (e.g., due to breathing). A prime candidate with the potential of fulfilling these demanding requirements is Magnetic Resonance Imaging (MRI). Due to its excellent soft-tissue contrast, MRI has proven to be the preferred imaging modality for the delineation of anatomical structures in RT. Other significant benefits of MRI include its capability for multiplanar and fast imaging as well as its non-invasive technique for generating images (i.e. no harmful radiation is delivered to patient).